I don't understand the question. What is the claimed analogy between the two?
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ShreevatsaRJan 29 '11 at 5:43

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@ShreevatsaR: I guess the question is why we write $1 and not 1$, but 100% and not %100. It's about unit placement.
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RegDwigнt♦Jan 29 '11 at 5:44

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@RegDwight: But % is not a unit; it's dimensionless and just means "divided by 100". And most units are placed after the number anyway — "5 cm", "320 Hz" etc. — so it's not clear why the question is asked about %. Perhaps the question is about why "$" is special in that it can go before the number, but until the questioner clarifies we can't be sure. :-)
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ShreevatsaRJan 29 '11 at 6:03

Different conventions apply to currency symbols and other types of symbol. English contains many conflicting conventions, in many cases because the conflicting conventions were each inherited from a different source. The USA currency prefix position was probably influenced by that of the British currency prefix which I suspect came from the Romans and might reflect Roman conventions or Latin word ordering. The positioning of abbreviations for temperature and other units of measure may have come from other sources.

% (Percent) is french, meaning if you had 5%, you're actually saying "5 per hundred", as cent means hundred in french. So if you're saying 5 in 100 people are born blind, that's "5 per hundred people are born blind", (5%) where as saying %5 would be "Per hundred 5 people are blind", which makes less sense.